The use of lithium titanate Li4Ti5O12, or lithium titanium spinel for short, as a substitute for graphite as anode material in rechargeable lithium-ion batteries was proposed some time ago.
A current overview of anode materials in such batteries can be found e.g. in Bruce et al., Angew. Chem. Int. Ed. 2008, 47, 2930-2946.
The advantages of Li4Ti5O12 compared with graphite are in particular its better cycle stability, its better thermal rating and the higher operational reliability. Li4Ti5O12 has a relatively constant potential difference of 1.55 V compared with lithium and achieves several 1000 charge/discharge cycles with a loss of capacity of <20%.
Thus lithium titanate has a clearly more positive potential than graphite which has previously usually been used as anode in rechargeable lithium-ion batteries.
However, the higher potential also results in a lower voltage difference. Together with a reduced capacity of 175 mAh/g compared with 372 mAh/g (theoretical value) of graphite, this leads to a clearly lower energy density compared with lithium-ion batteries with graphite anodes.
However, Li4Ti5O12 has a long life and is non-toxic and is therefore also not to be classified as posing a threat to the environment.
Recently, LiFePO4 has been used as cathode material in lithium-ion batteries, with the result that a voltage difference of 2 V can be achieved in a combination of Li4Ti5O12 and LiFePO4.
Various aspects of the preparation of lithium titanate Li4Ti5O12 are described in detail. Usually, Li4Ti5O12 is obtained by means of a solid-state reaction between a titanium compound, typically TiO2, and a lithium compound, typically Li2CO3, at high temperatures of over 750° C. (U.S. Pat. No. 5,545,468). This high-temperature calcining step appears to be necessary in order to obtain relatively pure, satisfactorily crystallizable Li4Ti5O12, but this brings with it the disadvantage that primary particles are obtained which are too coarse and a partial fusion of the material occurs. The product obtained in this way must therefore be ground extensively, which leads to further impurities.
Typically, the high temperatures also often give rise to by-products, such as rutile or residues of anatase, which remain in the product (EP 1 722 439 A1).
Sol-gel processes for the preparation of Li4Ti5O12 are also described (DE 103 19 464 A1). In these, organotitanium compounds, such as for example titanium tetraisopropoxide or titanium tetrabutoxide, are reacted in anhydrous media with for example lithium acetate or lithium ethoxide to produce Li4Ti5O12. However, the sol-gel methods require the use of titanium starting compounds that are far more expensive than TiO2 and the titanium content of which is lower than in TiO2, with the result that preparing a lithium titanium spinel by means of the sol-gel method is usually uneconomical, in particular as the product still has to be calcined after the sol-gel reaction in order to achieve crystallinity.
In addition, preparation processes by means of flame spray pyrolysis are proposed (Ernst, F. O. et al. Materials Chemistry and Physics 2007, 101 (2-3) pp. 372-378) as well as so-called “hydrothermal processes” in anhydrous media (Kalbac, M. et al., Journal of Solid State Electrochemistry 2003, 8 (1) pp. 2-6).
Further possibilities for preparing lithium titanate, in particular by means of solid-state processes, are for example described in US 2007/0202036 A1 and U.S. Pat. No. 6,645,673, but they have the disadvantages already described above, that impurities such as for example rutile or residues of anatase are present, as well as further intermediate products of the solid-state reaction such as Li2TiO3 etc.
Furthermore, in addition to the preparation of non-doped Li4Ti5O12, the preparation and properties of Al-, Ga- and Co-doped Li4Ti5O12 have also been described (S. Huang et al. J. Power Sources 165 (2007), pp. 408-412).